The invention relates to apparatus for treating substrates, in which the latter are immersed for some time in a bath containing a liquid and are then taken therefrom so slowly that practically the whole quantity of liquid remains in the bath.
Apparatus of this kind may be used in the manufacture of electric circuits on all kinds of substrates, such as, for example, integrated circuits on semiconductor wafers (for example of silicon), drives for liquid crystal displays on transparent plates of glass or quartz or circuits on plates of synthetic material (circuit boards). The method may also be used in the manufacture of shadow masks for television picture tubes or in the manufacture of CD or VLP records. In all these cases, the substrates are immersed many times for some time in a bath containing a liquid, for example in galvanic baths for deposition of metals, in etching baths for etching patterns into metal layers or into semiconductor material, in development baths for developing exposed photolacquer layers and in rinsing baths for cleaning the substrates. After treatment in the liquid baths, the substrates are taken from the liquid and are dried. The substrates can be taken from the liquid by lifting or withdrawing from the liquid, and by causing the liquid to flow out of the bath.
U.S. Pat. No. 4,722,752 discloses a method in which silicon wafers are immersed for some time for cleaning in a liquid containing hot (90xc2x0 C.) deionized water. Subsequently, the silicon wafers are lifted from the water slowly (at a speed of about 5 cm/min. This speed is sufficiently low to ensure that the silicon wafers leave the bath in practically dry state. The surface tension then should play a part.
It has been found in practice that in this method of drying contaminations from the liquid can remain on the substrates. It has been found that with the known method described a water film having a thickness of a few microns remains on the silicon wafers. This water film then evaporates rapidly, but this may give rise to so-called xe2x80x9cdrying marksxe2x80x9d. During drying, the water film can be contracted so that contaminations in the water, which are often of organic or metallic nature, remain in local concentration on the wafers. Such residues can be very disturbing if, for example, the wafers must still be subjected to an etching treatment. They can locally delay the etching treatment or even prevent this treatment. Further, dust particles present in the water film also remain on the wafers.
The invention has inter alia for its object to provide a method, in which the said disadvantages are obviated.
For this purpose, according to the invention, the method mentioned in the opening paragraph is characterized in according to the invention the substrates are brought directly from the liquid into contact with a vapour, not condensing thereon, of a substance which is miscible with the liquid and yields, when mixed therewith, a mixture having a surface tension lower than that of the liquid. It is then a surprise to find that, after the substrates have been dried in this manner, no drying marks and contaminations are present any longer on these substrates. Further, experiments have shown that, if nevertheless a liquid film remains on the substrates, this film must be thinner than 3 nm.
It is presumed that the much more satisfactory drying to the invention, in which apparently a much smaller quantity of liquid or no liquid remains on the substrates, is obtained by the Marangoni effect. If a lyophilic substrate is partly immersed in a liquid, the liquid constitutes at the substrate a concave meniscus. A liquid film having a thickness increasing in the direction of the liquid bath is then present on the substrate. If such a liquid film is brought into contact with a vapour not condensing on the substrate of a substance miscible with the liquid, this substance will be mixed with the liquid in the film in such a manner that its concentration therein initially decreases in the direction of the liquid bath. The concentration of the substance then exhibits a gradient in the liquid film. Since, when mixed with the liquid, the substance yields a mixture having a surface tension which is lower than that of the liquid, the gradient in the concentration will result in a gradient in the surface tension in the liquid film. By this gradient an additional force is exerted on the liquid film in the direction of the liquid bath (Marangoni effect). Thus, a more satisfactory drying of the substrates is obtained.
According to the invention, the vapour does not condense on the substrates. If this should be the case, the substrates would be covered, after having been taken from the liquid, with a layer of condensed vapour. Of course, such a layer must also be removed, as a result of which the drying process will take more time. Further, such a layer can attack the substrate. This is the case, for example, if the layer contains an organic solvent and if the substrate is provided, for example, with a photolacquer pattern. In practice, also more dust particles will remain on the substrates after drying thereof. An experiment with a vapour condensing on the substrates showed that about ten times more dust particles were present on the substrates than with a vapour not condensing thereon. A vapour not condensing on the substrates has a vapour pressure which is not saturated at the temperature of the bath and the substrates, whereas a vapour condensing on the substrates indeed has such a saturated vapour pressure.
As already stated above, substrates can be treated in different liquid baths. In practice, however, these baths mostly contain water. In this case, preferably an organic solvent is used as a substance miscible with the liquid. It has been found that many alcohols, glycols, aldehydes, esters and ketones, and also a solvent, such as tetrahydrofurane, can ensure that the substrates are dried satisfactorily. The invention can also be utilized successfully if the baths contain other liquids, for example, alcohol. The substrates may be brought directly from alcohol into contact with the vapour of the organic solvent 1,1,1,-trifluorotrichloroethane in order to obtain a satisfactory drying.
If the bath in which the substrates are immersed for some time contains water, the vapor is preferably made of an organic solvent having a solubility in water which is higher than 1 g/l and having a vapour pressure lying between 25 and 25,000 Pascal. Experiments have shown that in these conditions satisfactory drying results are obtained. If the solubility in water is lower than the said amount, evidently too little vapour is taken up by the liquid to cause a surface tension gradient which is sufficiently large to obtain the desired drying. If the vapour pressure is lower than the lower limit, evidently a sufficient quantity of vapour is not taken up by the liquid either. If the vapour pressure is higher than the upper limit, such a large quantity of vapour is taken up by the liquid that also in this case evidently a surface tension gradient is obtained which is too small to obtain the desired drying.
The substrates can be taken from water comparatively rapidly (at a speed up to 1.5 cm/sec) if according to the invention an organic solvent is used from the group comprising ethyl glycol, 1-propanol, 2-propanol and tetrahydrofurane.
If substrates covered with a layer of photolacquer are taken from water (for example after development), 2-propanol is preferably used as organic solvent. The photolacquer is practically not attacked by this vapour.
Preferably, the substrates are brought into contact with the vapour by mixing the vapour with a carrier gas and by passing this mixture over the liquid. Thus, the vapour can be brought locally and in high concentration into contact with the substrates. The pressure of the vapour can then be chosen so that the condensation thereof on the substrates is readily avoided.
When substrates, for example silicon wafers, are taken from a liquid and are brought into contact, with a suitable vapour above the liquid by means of a lifting member present in the liquid, those parts which then project above the liquid will be dry. If the substrates are then gripped at these dry parts and are further lifted out of the liquid, a drop of liquid can stick to those parts of the substrates which are the last to leave the liquid. Such a drop of liquid at the edge of an otherwise dry substrate need not be objectionable, but may give rise to problems already mentioned above. It is a surprise to find that the formation of such drops is avoided if, those parts which are the last to leave the liquid are supported when leaving the liquid by a knife-shaped member. The drops then flow away to this knife-shaped member. It is particularly practical if the substrates are lifted from the liquid by means of the knife-shaped member.
The invention further relates to carrying out the method mentioned in the opening paragraph, apparatus provided with a lifting member for lifting the substrates above the liquid and with means for gripping the dry substrates above the liquid.
Such an apparatus is known from U.S. Pat. No. 4,722,752, in which the lifting member lifts the substrates partly out of the liquid, whereupon the substrates are gripped at their dry parts and are further lifted from the liquid by a substrate cassette. The lifting member then remains under the liquid so that wet parts of the substrates, when leaving the liquid, are in contact neither with the lifting member nor with gas the substrate cassette. At such contact areas drops of liquid could be left behind.
In order to obtain a satisfactory drying of the substrates, the latter are brought into contact, as described above, immediately upon leaving the liquid with a vapour not condensing thereon of a substance which, when mixed with a liquid, yields a mixture having a surface tension lower than that of the liquid. For this purpose, the arrangement according to the invention is provided with leads with outlet openings to bring this vapour into the desired contact with the substrates.
On the lower side of the substrates which in the known arrangement are taken from the liquid, drops of liquid stick. In order to avoid this, a knife-shaped member, supports the substrates at those parts of the substrates which are the last to leave the liquid. The drops then flow away via the knife-shaped member.
Preferably, the knife-shaped member is provided on the lifting member so that the substrates are lifted from the liquid by the knife-shaped member. Thus, a practical and simple arrangement is obtained.
The invention will now be described more fully, by way of example, with reference to a drawing and a few embodiments. In the drawing: